Brain mitochondrial ATP-insensitive large conductance Ca+ 2-activated K+ channel properties are altered in a rat model of amyloid-β neurotoxicity

Abstract

Mitochondrial dysfunction is a hallmark of amyloid-beta (Aβ)-induced neuronal toxicity in Alzheimer's disease (AD). However, the underlying mechanism of how Aβ affects mitochondrial function remains uncertain. Because mitochondrial potassium channels have been involved in several mitochondrial functions including cytoprotection, apoptosis and calcium homeostasis, a study was undertaken to investigate whether the gating behavior of the mitochondrial ATP- and ChTx-insensitive-IbTx-sensitive Ca2+-activated potassium channel (mitoBKCa) is altered in a rat model of Aβ neurotoxicity. Aβ1–42 (4μg/μl) was intracerebroventricularly injected in male Wistar rats (220–250g). Brain Aβ accumulation was confirmed two weeks later on the basis of an immunohistochemistry staining assay, and physiological impacts measured in passive avoidance task cognitive performance experiments. Brain mitochondrial inner membranes were then extracted and membrane vesicles prepared for channel incorporation into bilayer lipid. Purity of the cell fraction was confirmed by Western blot using specific markers of mitochondria, plasma membrane, endoplasmic reticulum, and Golgi.Our results first provide evidence for differences in mitoBKCa ion permeation properties with channels coming from Aβ vesicle preparations characterized by an inward rectifying I–V curve, in contrast to control mitoBKCa channels which showed a linear I–V relationship under the same ionic conditions (200mM cis/50mM trans). More importantly the open probability of channels from Aβ vesicles appeared 1.5 to 2.5 smaller compared to controls, the most significant decrease being observed at depolarizing potentials (30mV to 50mV). Because BKCa-β4 subunit has been documented to shift the BKCa channel voltage dependence curve, a Western blot analysis was undertaken where expression of mitoBKCa α and β4 subunits was estimated using anti-α and β4 subunit antibodies. Our results indicated a significant increase in mitoBKCa-β4 subunit expression coupled to a decrease in the expression of α subunit. Our results thus demonstrate a modification in the mitoBKCa channel gating properties in membrane preparations coming from a rat model of Aβ neurotoxicity, an effect potentially linked to a change in mitoBKCa-β4 and -α subunits expression or increased ROS production due to an enhanced Aβ mitochondrial accumulation. Our results may provide new insights into the cellular mechanisms underlying mitochondrial dysfunctions in Aβ neurotoxicity.